1. Field of the Invention
The invention generally relates to a force measurement system and a method of calibrating the same. More particularly, the invention relates to a force measurement system and a method of calibrating the same that is capable of more accurately determining an applied load.
2. Background and Related Art
The use of strain gages in load transducers to measure forces and moments is a known art. A transducer can incorporate one or more load channels. Each load channel measures one of the load components, and is comprised of one or more strain gages mounted to one or more elastic elements that deform under the applied load. An appropriate circuitry relates the resistance change in each set of gages to the applied force or moment. Strain gages have many industrial, medical, and electrical applications due to their small size, low production cost, flexibility in installation and use, and high precision.
A typical low profile, small, multi-component load transducer only functions correctly when the axial (i.e. vertical) force acts relatively central to the transducer. Specifications of such transducers indicate a maximum allowable offset for the force being approximately half the diameter of the transducer. Technical specifications of transducers are given as the allowable force and moment ratings, where the moment rating is obtained by multiplying the maximum allowable force with the maximum allowable offset of the force.
Transducers can be used to measure forces and moments in linkages such as those found in a robotic arm, where the links are connected by joints, and the magnitude and offset of the forces transmitted by these joints are used to control the linkage. In such applications, it is desirable to have a transducer which has significantly higher moment capacity than those available in the market. Accordingly, there is a need for an improved multi-component, low profile load transducer with high moment capacity.
When conventional load transducers are utilized in conjunction with force plates, unique load transducers must be designed and fabricated for force plates having a particular footprint size. Consequently, in order to fit force plates with varying footprint sizes, many different custom load transducers are required. These custom load transducers significantly increase the material costs associated with the fabrication of a force plate. Also, conventional load transducers often span the full length or width of the force plate component to which they are mounted, thereby resulting in elongate load transducers that utilize an excessive amount of stock material.
Therefore, what is needed is a load transducer that is capable of being interchangeably used with a myriad of different force plate sizes so that load transducers that are specifically tailored for a particular force plate size are unnecessary. Moreover, there is a need for a universal load transducer that is compact and uses less stock material than conventional load transducers, thereby resulting in lower material costs. Furthermore, there is a need for a force measurement assembly that utilizes the compact and universal load transducer thereon so as to result in a more lightweight and portable force measurement assembly.
Also, certain strain gages or strain gage pairs of a typical multi-component load transducer are configured to be sensitive to a particular component of the applied load (i.e., to a particular one of the force or moment components being measured). However, because the body portion of a typical load transducer has unavoidable machining imperfections, and the strain gages are not perfectly positioned on the body of the load transducer, there is some crosstalk between the channels of the load transducer. For example, a channel that is intended to be sensitive only to the x-component of the force may also emit a non-zero output signal when only a vertical force is applied to the load transducer (i.e., when the z-component of the force is applied). Thus, in such a typical load transducer, there is undesirable crosstalk between the channels.
Moreover, the output of a typical multi-component load transducer is also undesirably affected by the ambient temperature of the environment in which the load transducer is disposed. For example, the accuracy of a load output signal of a load transducer that is disposed in a space having a high ambient temperature (e.g., a space with a temperature of 140 degrees or more) will be adversely affected by the high ambient temperature. That is, high ambient temperature will introduce inaccuracies in the load output signal.
Furthermore, the position of the applied load may also adversely affect the accuracy of the output signal of a typical multi-component load transducer. For example, when the load is applied at a location that is near the periphery of the measurement surface of the load measurement device in which the load transducer is installed, the load output of the load transducer is often less accurate than when the load is applied proximate to the center of the measurement surface of the load measurement device. As such, the measurement accuracy of a typical load measurement device undesirably varies depending upon the position of the load applied thereto.
Therefore, what is also needed is a load transducer system that is capable of correcting the output signal of a load transducer so as to reduce or eliminate the effects of crosstalk among the channels of the load transducer. In addition, there is a need for a load transducer system that is capable of correcting the output signal of a load transducer so as to reduce or eliminate the effects of changes in temperature on the output of the load transducer. Further, there is a need for a load transducer system that is capable of accurately determining the applied load regardless of the location of the applied load being measured by the load transducer.
Further, force plates historically have been calibrated by applying known loads at known locations and using the collected data to form a calibration matrix. This unique calibration matrix, stored on the force plate, converts the raw signal input into a calibrated force output. This methodology provides a global calibration for the force plate. However, the global calibration of the force plate can result in unacceptable errors for certain regions of the force plate (e.g., near the edges of the force plate), and can also result in unacceptable errors for force plates having non-standard shapes (e.g., force plates with top plate components having shapes other than a rectangular shape).
Therefore, what is additionally needed is a force measurement system that allows for more versatile transducer designs and minimizes measurement errors. Moreover, a force measurement system is needed that is capable of correcting for load measurement errors resulting from loads applied near the periphery of the force measurement assembly. Furthermore, a need exists for a load calibration process for a force measurement system that results in more accurate load measurements by correcting the computed load based upon the applied position of the load.